I too have the exact same problem!
with the old firmware everything was ok, now with version 3.0.1 the sonoff is online but you can't change the switch!
At me not solved. Basic is at 3.0.1, Ewelink at 3.6.1
It is possible to switch the basic via a scene from a T1-switch. It is possible to switch the basic via Google home and Alexa. But via ewelink I can do nothing.